what are phantom traffic jams?
if you have driven in rush hour traffic coming into or leaving a city then you have likely experienced stop and go traffic, even when there isn’t an accident or lane closure creating a physical bottleneck. this occurs when there is a high enough density of traffic. past a specific density, anyone hitting the brakes can propagate a wave of braking which grows in intensity and travels backwards through traffic from the point of origin. these traveling traffic slowdowns are sometimes called phantom traffic jams.
how can you attenuate them?
in principle it really is simple; drive in a way that dissuades the car behind you from braking. that’s it.
you may be able to intuit how you might achieve this, but lets go through the list of sub-goals which follow from that main goal. you should avoid braking; the main thing which will cause the car behind you to brake is you braking. how do you avoid braking? you need to leave enough of a gap between you and the car ahead of you that (when it hits one of the slowdown waves propagating towards you) you are able to slow down sufficiently to not hit it without braking. this distance will vary greatly depending on traffic conditions. the way i think about it is that while in the heavy traffic area there are three speeds. there is the higher speed you would naturally go when you are between slowdown waves, there is the slower speed you would naturally go when you are in a wave, and there is the equilibrium speed. when in stop and go traffic i seek to drive the equilibrium speed (leaving a gap in front of me which is sufficient to maintain that speed).
there are additional benefits to driving this way while in heavy traffic. it is better for your car to not be using the brakes constantly, burns less gas (every bit of your momentum you give up by braking is fuel you wasted) and is (i find) a less stressful way to drive once you are used to it. beyond the benefits for you as an individual, the size of gap you will find yourself needing to leave in front of you also helps improve traffic in other ways. if you leave a meaningful gap in the slow lane then cars merging onto the road will be able to do so without causing the kind of abrupt braking which spawns new waves. the gap directly lowers the density of traffic for a local area, allowing the cars around you to change lanes and sort themselves by speed without creating snags. of course, as cars use the gap you are maintaining, you will need to ensure that gap is preserved - ideally without braking.
join us in driving with compassion
i think it is fair to say that few people enjoy being caught in stop and go traffic. driving in the way i have outlined makes coming across a traffic jam almost exciting for me. if you are ever fortunate enough to drive this way and crest a hill, seeing snarled traffic ahead and slower but noticeably smoother traffic behind you can feel extremely rewarding. of course, in the same way that driving calmly can have a significant positive effect, a few particularly aggressive drivers can undo it all. that is why i and other people who choose to drive this way in traffic jams need your help. since most people drive on mental auto-pilot, having a few more actively helpful drivers and a few less actively harmful drivers really can make a huge difference to so many people.